1. Field of the Invention:
This invention relates to a method for the production of open-cell foams of cross-linked polyolefins.
2. Description of the Prior Art:
Heretofore, for the production of an open-cell foam of a cross-linked polyolefin, the method which comprises partially decomposing a foaming agent and a cross-linking agent in a foamable and cross-linkable polyolefin composition in a tightly closed metal mold under application of pressure, decomposing the remainders of the foaming agent and cross-linking agent under normal atmospheric pressure to obtain a closed-cell foam, and subsequently compressing the closed-cell foam thereby rupturing the closed cells in the closed-cell foam has been proposed (for example, see Japanese Patent Publication No. SHO 59-23545 and Japanese Patent Application laid open to public inspection, KOKAI No. SHO 56-146732). In the method of this sort, since the decomposition of foaming agent and cross-linking agent is effected by heating the composition in the tightly closed metal mold under application of pressure, the cross-linking reaction of polyolefin takes place but the formation of cells is suppressed, and the expansion of polyolefin occurs only after the release of applied pressure. That is, said method first induces the cross-linking reaction and thereafter incites the foaming. Since the so-called "preceding cross-linking" proceeds in the production of closed-cell foam, the membranes of the produced cells are so tough as to render difficult the rupture of the cell membranes and the creation of intercommunication among the cells of the foam. Thus, by this method a foam having an open-cell ratio of 100% or close to 100% could not be attained.
U.S. Pat. No. 4,435,346 to Ito et al. assigned to the assignee of this application and Japanese Patent Publication No. Hei 1-44,499 filed by the same assignee disclose methods for the solution of the problem mentioned above. These methods basically comprise thermally shaping a foamable and cross-linkable polyolefin composition into a desired shape, then heating the shaped composition under normal atmospheric pressure thereby concurrently decomposing a cross-linking agent and a foaming agent contained in the composition and consequently giving rise to a foam, and subsequently mechanically deforming the resultant closed-cell foam thereby establishing intercommunication among the cells of the foam. In accordance with these methods, an open-cell foam of a cross-linked polyolefin having an open-cell ratio of 100% or close to 100% can be produced.
Since these methods require the shaped composition which has not been cross-linked to be thermally foamed under normal atmospheric pressure, namely in a metal mold which is not kept under an airtight condition, however, when the shaped composition is rapidly foamed, the produced foam is cracked by the friction between the composition in the process of foaming and the inner surface of the metal mold. For the purpose of solving this problem, in the production of an open-cell foam having a large thickness exceeding 100 mm, for example, the thermal foaming should be carried out mildly over a period of at least 100 minutes. As a result, the production suffers from inferior operational efficiency and the thickness of an unfoamed surface skin layer and an ununiformly foamed part occurring near the surface skin increases due to the protracted application of heat. Since these parts must be cut off the foam, the product suffers from poor material utilization. Further, the produced foam tends to betray a serious dispersion of cell diameter and physical properties. Thus, the methods under discussion have room for further improvement.